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Nanonets Snare Energy

A new material could cheaply convert sunlight into hydrogen.
September 10, 2008

One problem with solar cells is that they only produce electricity during the day. A promising way to use the sun’s energy more efficiently is to enlist it to split water into hydrogen gas that can be stored and then employed at any time, day or night. A cheap new nanostructured material could prove an efficient catalyst for performing this reaction. Called a nanonet because of its two-dimensional branching structure, the material is made up of a compound that has been demonstrated to enable the water-splitting reaction. Because of its high surface area, the nanonet enhances this reaction.

Net reaction: Nanonets, structures made up of branching titanium and silicon wires, are flat yet have a high surface area, making them more efficient at using solar energy to split water into oxygen and hydrogen fuel. The top image shows a nanonet magnified 50,000 times. At bottom, a flexible nanonet rolls up when poked by the tip of a scanning tunneling microscope. Both images were taken with a tunneling electron microscope.

Researchers led by Dunwei Wang, a chemist at Boston College, grew the nanonets, creating structures made up of branching wires of titanium and silicon. Last year, researchers at the Max Planck Institute, in Germany, showed that titanium disilicide, which absorbs a broad spectrum of visible light, splits water into hydrogen and oxygen–and can store the hydrogen, which it absorbs or releases depending on the temperature. Other semiconducting materials have been tested as water-splitting catalysts but have proved unstable.

Wang set out to increase the surface area of titanium disilicide in the hope of improving its performance: “More contact means higher efficiency,” he explains. Wang says that the nanonets are one of the most structurally complex, two-dimensional nanomaterials yet made. “When I want to make something small, I have to restrain the growth,” he says. Making long, thin structures like nanowires and nanotubes requires limiting growth in all but one dimension. Limiting growth in one dimension while promoting the growth of complex structures in the other two dimensions is more challenging, says Wang. But under the right conditions, he found, it happens spontaneously.

The nanonets, made up of flexible wires about 15 nanometers thick, grow spontaneously from titanium and silicon flowing through a reaction chamber at high temperatures. In a paper in the journal Angewandte Chemie, Wang’s group describes the synthesis of nanonets. The material is 10 times more electrically conductive than its bulk form. Conductivity is an important property for water-splitting catalysts. Wang says that he has tested the nanonets’ water-splitting properties, although this work has not yet been published. In preliminary tests, the nanostructured version of the material performs about 100 times better than bulk titanium disilicide.

However, says Peidong Yang, a chemist at the University of California, Berkeley, the method Wang has used to fabricate the nanonets may limit their usefulness. When researchers first began trying to make complex, two-dimensional nanostructures, says Yang, the main application they had in mind was electronics. Although the bulk version of the material is commonly used to form electrical contacts on microprocessors, Wang’s nanostructured material will not be, predicts Yang. For electronics applications, being able to rationally design the material’s structure is important, he says. “You want to make a branch here, not there,” says Yang. So although their planar structure would be compatible with flat devices, the nanonets are unlikely to be found on future microprocessors.

Yang agrees, however, that Wang’s nanonets have good surface area and conductivity and “might be useful as an electrode for water splitting.” However, nanonets will be entering a crowded field, with many researchers and companies developing such technologies.

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